I'm not aware of a single source for this information—typically any contaminant would constitute damage as others have mentioned and so why bother figuring out exactly what it is. With that said, here are a few things that I recall seeing on coins posted here at CCF: tape residue, dried glue/adhesive, ink, paint, fingernail polish, plating, solder, braze, PVC residue, varnish, tarnish, rust stains, and enamel.
I'm sure my list is woefully small, but hopefully you get the idea.
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz